Characteristics / Formation of Extrusive Landform Feature:

Diagram and Named Examples of Features:

Acid / Lava Dome Volcano

Characteristics:

• dome shapped accumulations of volcanic rock
• these features have steep, convex sides;
• narrow base and high cones
• may have secondary cones;

Formation of:

• steep, convex sides due to acidic, andesitic lava, which is too viscous to flow far from the source and it soon cools and solidifies
• domes, often plug vents as lava solidifes, these may lead to the formation of a secondary vent on the side of the volcano due to the build up of pressure
• due to the plugging of vents, gasses are trapped and eruptions are often explosive in nature.

Common at destructive margins

Examples:

Lassen Peak Volcanic Dome (NW USA)

Shield Volcano

Characteristics:

• gentle sloping cones
• wide base and not particularly high
• consists of layers of basaltic lava

Formation of:

• built up due to succesive flows of basaltic lava
• due to the low viscosity and high temperatures, the lava flows over wide distances away from the vent;
• the lava gradually cools as thin, gently dipping sheets of volcanic rock

Common at constructive margins and hot spots (e.g. Hawiian Islands)

Examples:

Medicine Lake Shield Volcano (Cascades)

Hawaiian Islands - e.g. Mauna Loa

Cinder Cone

Characteristics:

• Slightly concave sides
• consist of successive layers of material (ash and cinders)

Formation of:

• largest materials fall closest to the summit forming steep but stable sides close to the vent
• finer materials are carried further away and form gentle slopes at the base of the cone.

Examples:

Lava Butte, Oregon (Cascades)

Composite Cone / Stratovolcano

Characteristics:

• often very large structures with steep and fairly symmetrical sides;
• consist of alternating layers of acidic lava and ash;
• often has parasitic or secondary cone (several are common due to earlier eruptions);

Formation of:

• These cones are associated with andesitic magma, during violent eruptions, when the vent has been blocked, ash and other pyroclasts are produced during the eruption;
• In subsequent less explosive eruptions, lava may be released - giving rise to the alternating layers of lava and ash;
• these cones hava conduit system through which magma rises to the surface, solidfied lava in the conduit system helps to strengthen the cone;
• Where the passage of magma becomes blocked and pressure builds, magma may flow from fissures on the volcanoes flank, forming parasitic or secondary cones.

Common at destructive boundaries (subduction zones)

Examples:

Mount St Helens, Washington State (Cascades)

Caldera

Characteristics:

• large. steep walled, basin shaped depressions;
• ofen have a lake (or lagoon, if below sea-level) inside the crater
• may have new volcanic cones growing inside;

Formation of:

• Formed due to highly explosive volcanic activity - fresh andesitic magma entering and filling the magma chamber, may trigger a highly explosive volcanic (eruption) (diagram 2)
• As the eruption progresses, the magma chamber gradually empties and there is no magma left to support the roof of the chamber. The mountain summit therefore collapses into the chamber creating a caldera (earth movements may cause further subsidence of the sides); (diagram 3)
• The bowl-shaped depression often fills with water, forming a lake (or lagoon, if below sea-level);
• If new magma continues to rise, small, new volcanic cones may form within the caldera (e.g. Crater Lake)

After 1000s of years, fresh magma may enter the magma chamber causing it to re-inflate and the caldera floor to dome up as a Resurgent Caldera - when these erupt, they can be the most destructive natural phenomenum on earth. They form as large amounts of highly viscous and explosive rhyoltic magma rises to the surface.

Examples:

Crater Lake Caldera, Oregon

Lava Plateaux

Characteristics:

• Extensive, flat / very gentle sloping,
• Layered structure (layers of lava from series of eruptions over time)
• flat, featureless (usually limited soil / vegetation cover)

Formation of:

• Lava Plateaux are formed during fissure eruptions, where basalt lavas (known as flood basalts) pour out of cracks in the surface (fissures) rather than from a central vent;
• as the lava is runny (low viscosity) and takes a long time to cool, it travels long distances from the fissure;
• these features are layered structures caused by the accumulation of lava over a series of lava flows, forming as broad plateaux rather than piling up as a volcanic mountain;
• slow cooling can lead to the formation of columnar jointing;
• Rivers often cut across the lava plateau, forming gorges - for example the Columbia river which has cut across the Columbia Plateau

The Deccan Plateau is a huge expanse of lava covering an area of 700,00km2 and consisting of 29 lava flows which have occured within the last 2 million years. A number of river valleys (including the Wardha river) are eroded into the plateau which lies at 700-900m above sea level.

Examples:

Columbia Plateau - NW USA (Cascades area)

Deccan Plateau - NW India